This invention concerns the stripping of metal coatings from metal substrates such as copper, zinc, and their alloys. More particularly, it concerns the stripping of metal coatings (i.e., nickel, chromium, tin, lead, tin/lead alloy, cobalt, cadmium, or aluminum) in an electrolytic process employing an aqueous alkanesulfonic acid solution of 1 to 4 carbons as the electrolyte at a concentration of about 45 to about 70 weight percent.
Removal of metal coatings from metal substrates such as copper, zinc, and their alloys, is a common operation in metal finishing, either as a regular process step or as a remedial operation. For example, in printed circuit board manufacture, removal of solder resist is required as a standard procedure before precious metal is plated. In the manufacture of plumbing fixtures, defective electroplated nickel and chromium coatings need to be stripped and replated. In both cases, it is desired that the stripping process remove only the coating and leave the substrate intact and easily prepared for the next process step. The removal can be accomplished either by chemical immersion, usually in strong acid solutions, or by electrolytic stripping.
In chemical immersion, the workpiece is immersed in a solution formulated to remove the coating and leave the base metal coating free of corrosion until such time as the metal coating is completely removed. In electrolytic stripping, which is much faster, the work piece serves as the anode in an electrolytic bath through which a dc voltage is applied to remove the metal coating. Since the metal coatings are usually not uniform in thickness throughout the surface of the workpiece, it requires different lengths of time to remove the coating from different areas of the surface. Some portions of the substrate will be exposed to the electrolytic environment earlier than other portions and the earlier exposed surface would tend to be corroded unless the base metal is inert to the stripping solution. In order to prevent such unwanted corrosion, it is customary to add inhibitors. The function of the inhibitor is to prevent dissolution of the base metal in the electrolyte. The addition of inhibitor increases the complexity of the stripping process and may result in complicated procedures for maintaining the proper bath composition. Because of the toxic nature of many inhibitors, waste disposal often becomes a problem. Examples of commonly-used inhibitors that are toxic, are thioureas and chromic acid. Usually the formulations employed are very specific and will serve to strip only certain types of metal coatings from specific substrates. If an improper formulation is used, either no stripping will occur, or the substrate will be damaged by corrosion. A general method to strip several different metals from diversified substrates is therefore highly desirable.
Stripping metal coatings such as nickel, chromium, tin, lead, and tin/lead alloys from industrially-important, but easily-corroded, base-metal substrates such as copper, zinc, and their alloys, to provide a clean substrate surface, has been difficult to accomplish. Generally, additives are incorporated into the formulation to coat and protect the substrate surface. These additives must then be removed to provide the clean, non-corroded substrate surface required for further processing or refinishing. This requires adding an additional processing step and adds to cost.